1. Field of the Invention
This invention relates to image forming apparatus used in an electrophotographic printing or copoying machine, etc., employing toner to form a toner image on a sheet of paper. More particularly, the invention relates to a detachable fixing unit including rollers for pinching a sheet carrying a toner image thereon so as to fix the toner image on the sheet.
2. Description of the Related Art
In an electrophotographic recording apparatus a toner image is developed on a rotating photosensitive drum corresponding to an image to be recorded according to a well-known technique. A sheet of paper is fed into the apparatus and is brought into contact with the toner image so that the toner image is transferred onto the sheet while the sheet feed speed is synchronized with the speed of the rotating drum surface, according to a well-known technique. The sheet having the toner image thereon is then fed between a pair of parallel rotating rollers of a fixing unit. One of the rollers is generally a heat roller and the other is a backup roller. The rollers are pressed transversally together so that the toner is melted and fixed on the sheet. It has been a typical problem that the image bearing sheet being fed through the rollers will become jammed by the rollers. Therefore, many schemes have been devised for conveniently servicing the rollers, for example, to facilitate removal of the jammed sheets.
The fixing unit having rollers as outlined above may be constructed for axial detachment from a frame of the apparatus, as disclosed in U.S. Pat. No. 4,145,181. Of Edward et al. Problems of encountered in the use of the Edward et al. structure include the fact that the removal of the fixing unit from the frame for servicing is not efficient because the two rollers must be removed together with the jammed sheet pressed between the rollers, and the removal of the sheet is a troublesome operation. Additionally, the mechanism for pressing the rollers together must be included as part of the fixing unit resulting in higher initial cost as well as higher serving cost.
Alternatively, the rollers may be respectively on separate frames, and for servicing the frames are disengaged so as to release the pressure between the two rollers. Such apparatus is disclosed in Japanese Unexamined Patent Publication Sho No. 61-176964 by Fukushima, et al., and in U.S. patent application No. 890511 of Kawashima. The chief problem involved in the use of these structures is that the manipulation needed to remove the rollers from their respective frames for servicing of the rollers is inefficient.
As another previous alternative, the rollers are pressed together only when an upper chassis is engaged with its lower chassis, such as disclosed in Japanese Unexaminated patent publication Sho No. 62-299982 of Yamashita, et. al.. The main shortcoming of this structure is that the fixing unit is not detachable from the chassis, and accordingly the servicing of the rollers can not be efficiently carried out.